The spectral resolution or resolving power of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its power to resolve features in the electromagnetic spectrum. It is usually defined by
where is the smallest difference in wavelengths that can be distinguished, at a wavelength of . For example, the Space Telescope Imaging Spectrograph (STIS) can distinguish features 0.17 nm apart at a wavelength of 1000 nm, giving it a resolving power of about 5,900. An example of a high resolution spectrograph is the Cryogenic High-Resolution IR Echelle Spectrograph (CRIRES) installed at ESO's Very Large Telescope, which has a spectral resolution of up to 100,000.
The spectral resolution can also be expressed in terms of physical quantities, such as velocity; then it describes the difference between velocities that can be distinguished through the Doppler effect. Then, the definition is
where is the speed of light. The STIS example above then has a spectral resolution of 51 km/s.
- Kim Quijano, J., et al. (2003), STIS Instrument Handbook, Version 7.0, (Baltimore: STScI)